Rubbers vulcanizing systems

The Goodyear vulcanization process takes hours or even days to be produced. Accelerators can be added to reduce the vulcanization time. Accelerators are derived from aniline and other amines, and the most efficient are the mercaptoben-zothiazoles, guanidines, dithiocarbamates, and thiurams (Fig. 32). Sulphenamides can also be used as accelerators for rubber vulcanization. A major change in the sulphur vulcanization was the substitution of lead oxide by zinc oxide. Zinc oxide is an activator of the accelerator system, and the amount generally added in rubber formulations is 3 to 5 phr. Fatty acids (mainly stearic acid) are also added to avoid low curing rates. Today, the cross-linking of any unsaturated rubber can be accomplished in minutes by heating rubber with sulphur, zinc oxide, a fatty acid and the appropriate accelerator. 

Since these early days, the process and the resulting vulcanized articles have been greatly improved. In addition to NR, many synthetic rubbers have been introduced over the years. Furthermore, many substances other than sulfur have been introduced as components of curing (vulcanization) systems. 

The ZnCFO action as vulcanization active component of elastomeric compositions on the basis of rubbers of general and special assignment with various vulcanization systems is investigated. 

Despite of 150-year s history of vulcanization process, it is impossible to consider that fundamental and applied researches in direction of vulcanization systems perfection are completed. For today one of the ways of rubbers properties improvement is the synthesis and application of the new chemicals-additives, including, vulcanization active, that is connected, first of all, with reduction of global stocks of zinc ores as basic raw material for reception of traditional activator - zinc oxide. Besides, modem increase of industrial potential and the accumulation of big quantity wastes derivate the problems of ecological character, which require the emergency decision. Therefore creation of resourcesaving technologies of the new compounds reception from products of secondary raw material processing has paramount importance. 

An estimation of ZnCFO efficiency as vulcanization active component was carried out in modelling unfilled elastomeric compositions on the basis of isoprene, butadiene-nitrile, chloroprene and butyl rubbers of sulphur, thiuram, peroxide, metaloxide and resin vulcanization systems. 

Figure 7. Cure curves of vulcanization process of modeling unfilled elastomeric compositions on the basis of nitrile-butadiene rubber at 155°C with various vulcanization systems.

Influence of the ZnCFO contents (3,0 5,0 7,0 phr) on crosslink kinetics of the modelling unfilled rubber mixes from NBR-26 of sulfur, thiuram and peroxide vulcanization of recipe, phr NBR-26 - 100,0 sulfur - 1,5 2-mercaptobenzthiazole - 0,8 stearic acid - 1,5 tetramethylthiuramdisulfide - 3,0 peroximon F-40 - 3,0, is possible to estimate on the data of fig. 7. As it is shown, the increase of ZnCFO concentration results in increase of the maximum torque and, accordingly, crosslink degree of elastomeric compositions, decrease of optimum cure time, that, in turn, causes increase of cure rate, confirmed by counted constants of speed in the main period (k2). The analysis of vulcanizates physical-mechanical properties testifies, that with the increase of ZnCFO contents increase the tensile strength, hardness, resilience elongation at break and residual deformation at compression on 20 %. That is, ZnCFO is effective component of given vulcanization systems, as at equal-mass replacement of known zinc oxide (5,0 phr) the cure rate, the concentration of crosslink bonds are increased and general properties complex of rubber mixes and their vulcanizates is improved. 

The comparative estimation of efficiency of zinc oxide and ZnCFO similar concentrations (3,0 5,0 7,0 phr) as the agents of metaloxide vulcanization system was carried out on example of modelling unfilled elastomeric compositions from chloroprene rubber of recipe, phr chloroprene rubber - 100,0 magnesium oxide - 7,0. Kinetic curves of rubber mixes curing process at 155°C are shown on fig. 8. The analysis of the submitted data testifies, that at increase of zinc oxide contents vulcanization kinetics is changed as follows the scorch time and optimum cure time are decreased, the cure rate is increase. Vulcanization... 

Figure 11. Dependence of particles size of heterophase of modeling unfilled compositions on the basis of various rubbers with different vulcanization systems from the ZnO (a) or ZnCFO (b) contents...

It is possible to explain the decrease of ZnCFO efficiency as component of various vulcanization systems for rubbers of general and special assignment in the earlier submitted line (fig. 10) also by character of formed morphology of compositions. So, at use of ZnCFO as the activator of sulfur vulcanization the structure of rubbers with the minimal value of parameter r is formed, and at transition from sulfur to peroxide vulcanization of elastomeric compositions the particles size of heterophase is increased (fig. 11 b). 

Generalizing results of researches of the ZnCFO efficiency as the component of various vulcanization systems for rubbers of general and special assignment, it is possible to make the following conclusions ... 

ZnCFO is the effective vulcanization active component of the sulfur, thiuram, peroxide and metaloxide vulcanization systems for isoprene, nitrile-butadiene and chloroprene rubbers at the same time it is not effective in resin vulcanization system for butyl rubber. On a degree of positive influence on the properties of elastomeric compositions vulcanization systems with ZnCFO are arranged in a line ... 

In summaiy, theie aie a lange of vulcanizing systems which can be used for natural rubber, and the choice is dependent on the combination of properties required. No single one offers ideal, all-around properties combined with good heat resistance. The end user has to be selective, according to the properties required foi the final application. Certain properties such as oil resistance and gas permeability have been omitted from Table 3, because in legaid to these properties natural mbbei is substantially inferior to synthetic mbbers such as acrylonitrile rubber and halobutyl rubber (see Elastomers,... 

The basic compounding formulation specifies the minimum requirement of fillers, vulcanizing agents, and other substances that must be added to the rubber compound to achieve the desired properties. After the rubber, cure system and reinforcing filers have been selected it will be necessary to make several adjustments before all requirements are satisfied. It is generally sensible to start with the simplest mix formula for meeting the requirements. The recipe or the formula is usually written on the basis of hundred parts of rubber. For example if 5 parts of zinc oxide is to be added it is denoted as 5 phr (five parts per hundred rubber). Elementary compounding... 

Since in a normal vulcanization system at least part of the sulphur combines with the rubber during vulcanization, an obvious way of following vulcanization is to measure the decrease in free sulphur. This method is not used extensively since it is well known that the combination of free sulphur does not correlate well with the development of cross links or other physical properties. In addition to this the analytical procedure is lengthy and costly. However free sulphur determinations are often made on finished products as a means of checking for uniformity of the product and to estimate the degree of cure. Figure 8.6 below shows the rate of sulphur combination at different vulcanization temperatures for a typical... 

It can be seen that every type of vulcanization system differs from every other type in the kind and extent of the various changes that together produce the vulcanized state. In the vulcanization processes, consideration must be made for the difference in the thickness of the products involved, the vulcanization temperature and thermal stability of the rubber compound. The word cure to denote vulcanization is believed to have been coined by Charles Goodyear and the same has been a recognized term in rubber industry circles [2]. The conditions of cure will vary over a wide range according to the type of vulcanizate required and the facilities available in a rubber factory. Many factors must be predetermined, including the desired hardness of the product, its overall dimensions, the production turnover required and the pretreatment of the rubber stock prior to vulcanization. Hardness will normally be determined by the composition of the stock but it can also be influenced by the state of cure. 

Cross-links limit the mobility of macromolecular chains, and thus give rise to an increase in Tg. With a very low concentration of cross-links this effect is hardly detectable. Experiments with the system polystyrene + divinyl benzene showed that at a concentration of 0.4 % of DVB and higher a detectable increase of Tg occurred. With rubbers, vulcanized with sulphur, the detection limit for an increase of Tg appeared to be at 1 to 2 % of sulphur, so that for a technically vulcanized rubber the level of Tg is hardly influenced by the vulcanization process. 

Here is 1 or 2 in efficient vulcanization systems but may be as high as 8 under other conditions where cyclic and other structures are also formed in the reaction. The rubber article is essentially fixed in shape once it is vulcanized, and it is... 

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